Systems to locate mobile electronic devices are known generally as RTLS (Real Time Location Systems). One well-known example of RTLS technology is GPS (Global Positioning System) based technology. GPS is a satellite navigational system formed by 24 satellites orbiting the earth and corresponding earth-bound receivers on the earth. The GPS satellites continuously transmit digital radio signals that contain data on the satellites location and the exact time, determined by atomic clocks, to the receivers. Based on this information the receivers know the length of time it takes for the signal to reach the receiver and thus the distance of the satellite from the receiver. By calculating the distance to the satellite, the receiver determines that it is on the surface of an imaginary sphere centered at the satellite. Using three satellites, the GPS receiver can calculate its longitude and latitude based on where the three spheres intersect. Using four satellites, the GPS receiver can also determine its altitude.
The GPS receiver contains multiple electronic components, as well as an antenna for receiving the GPS signal. Until recently, GPS receivers were relatively large and expensive devices, which were used independently of other mobile electronic devices. The size of a typical large GPS receiver is on the order of over 100 cm2 and the cost ranges from a few hundred dollars to a few thousand dollars. This makes a large GPS receiver unable to be used in typical mobile electronic devices, for example, whose sizes are about equal to or smaller than the large GPS receiver and whose costs are usually substantially less than the GPS receiver.
With recent advances in electronics and antennas, however, the size and cost of the GPS receiver have been reduced dramatically. GPS receivers are now able to be incorporated into mobile electronic devices, as the cost has been reduced to about $50 or less. This is relatively inexpensive compared to the overall cost of the device. Examples of mobile electronic devices which have incorporated GPS receivers include laptop computers, cellular phones, and hand held personal digital assistants (“PDA”). However, although mobile electronic devices may incorporate such small, inexpensive GPS receivers often have problems receiving GPS signals indoors, unlike the larger, more expensive GPS receivers.
Thus, with indoor use of GPS receivers, cost and size problems exist with larger GPS antennas and more sophisticated GPS components. Similarly, with indoor use of GPS receivers, reception problems exist with smaller GPS antennas and less complex GPS components.